1. Field of the Invention
This invention relates to a method for starting a hydraulic turbine under a water head condition where there is experienced a considerably large variation in flow rate of water due to a variation in r.p.m. of a runner of a hydraulic turbine or pump turbine. (This will be referred to simply as a turbine.)
2. Description of the Prior Art
A turbine includes: a penstock adapted to introduce water from an upstream region to a downstream region; a runner which is directly coupled to the generator and rotates by water received from the penstock; two or more wicket gates for adjusting the flow rate of water running into the runner; and a draft tube adapted to introduce water from the runner to the downstream region. The flow rate of water running into the runner is adjusted by adjusting the opening of wicket gates i.e., by increasing or decreasing the opening thereof, thereby controlling the r.p.m. of the runner, i.e., the r.p.m. of a generator to a predetermined value.
Particularly in the case of a high-head pump turbine, there is noted a tendency or a phenomenon that, in case a given turbine operating point N1, i.e., N/.sqroot.H (N: r.p.m., H: effective head) is considerably large, a slight variation in effective head or r.p.m. directly leads to a large variation in flow rate of water, due to the action of centrifugal force, even though an opening of wicket gates is not varied. This tendency leads to a water hammer phenomenon in water tubes connected to the upstream and downstream regions, with the result that the turbine flow rate is varied. This establishes a cycle of such a phenomenon, i.e., a cyclic fluctuation of an output of the turbine. As a result, undesirable, noise is developed in a control system of a governor which controls the output and frequency of the generator by adjusting the opening of wicket gates. Accordingly, if the case comes to the worst, even the control function of the governor is lost, with the accompanying failure of synchronizing of the generator to the electric power transmission system.
Meanwhile, there arises a tendency at the present time that, from civil and/or economical viewpoints a long and narrow water tube is adopted, and a range of variation in water level is increased due to the use of smaller capacity upstream and downstream water reservoirs. In other words, many factors unfavorable to turbines from a viewpoint of water hammer phenomenon are being attended.
Stated differently, if these shortcomings in turbine characteristics are improved, and synchronizing of the generator may be accomplished satisfactorily, then further severe civil and/or economical requirements may be fulfilled.
The aforesaid background will be described in more detail in conjunction with FIG. 1, hereunder.
FIG. 1 is a graph representing a flow rate characteristic of a turbine, in which N1=N/.sqroot.H is along the X-axis and Q1=Q/.sqroot.H is along the Y-axis, as shown, with various openings of the wicket gates being respectively plotted as curves 1, 2, 3, 4, 5. In this respect, H represents head, Q represents flow rate, N represents r.p.m., N/.sqroot.H represents r.p.m. per unit head, and Q/.sqroot.H represents a flow rate per unit head. The opening of wicket gates is smallest as shown by a curve 1, and the opening of gates is increased in the order of curves 2, 3, 4, and 5. Assuming that an effective head H is constant, there is a tendency that turbine flow rate Q and, therefore Q1 is decreased with an increase in N and therefore a corresponding increase in N1, as shown by any one of wicket-gate opening curves 1, 2, 3, 4, 5. In this respect, a decreasing rate of the flow rate exhibits a sharp increase as N1 approaches to the rated N1 corresponding to the rated speed. In other words, under a constant rated r.p.m. (in general, a governor controls for the rated r.p.m. running), if a head is decreased, the turbine operating point is shifted from a relatively flat portion to a steep gradient portion of the curves.
Meanwhile, a no-load opening curve shows an upper limit of r.p.m. which may be achieved when increasing a wicket gate opening. In other words, as the r.p.m. is increased, a mechanical loss is increased, and water quantity is decreased due to a centrifugal force, with the resulting reduction in output of a turbine itself, so that the r.p.m. is saturated eventually. The no-load opening curve is a curve which represents saturated r.p.m.s at varying wicket gate openings. Meanwhile, in case the generator is accelerated and synchronized by a conventional means, the generator is in the first step, accelerated close to the rated r.p.m. (or synchronizing r.p.m.), then the wicket gates are controlled so as to get no load opening corresponding to the rated speed, and thereafter the wicket gates are maintained at an opening thus achieved. For instance, assume that N1 at a rated r.p.m. and at a certain head is N.sub.A (shown at A). The wicket gates are controlled so as to get an opening represented by a curve 3. Then synchronizing of the generator is commenced at a point Z, the intersection of the NA, curve 3, and the no-load opening curve.
In this respect, what is important is how to reach the point Z. FIG. 2 shows cases I and II, representing the conventional methods for starting the turbine/generator. According to the case I, the wicket gates are first opened to a so-called starting opening which is slightly larger than the no-load opening at a rated r.p.m. Then, the wicket gates are held in this condition. Meanwhile, when a turbine is started and accelerated to a given r.p.m., the governing action of a governor is commenced so as to automatically adjust the r.p.m. to a rated value, with the result that the wicket gates are brought down to and maintained at thereafter at the no-load opening at the rated r.p.m. and then the generator is synchronized there. According to this method, the opening of wicket gates is increased to an extent which is sufficiently larger than the no-load opening of wicket gates at the rated r.p.m., for rapid acceleration of the r.p.m. However, an excessively large opening of the wicket gates leads to overshooting of r.p.m., thereby increasing the time for matching of speeds. In contrast thereto, according to the case II, the wicket gates are opened gradually from their rest condition for increasing the r.p.m. gradually. In this case, the acceleration time is extended as compared with the case I, while the overshooting may be avoided, and may result in an eventual shortened starting time. However, in either case, as far as generators are run in a relatively gentle gradient region of curves, synchronizing of the generator may be accomplished in a satisfactory manner. However, this is not always true with the case where the turbine is run in the region of steep gradients of curves. If the aforesaid conventional methods for starting the turbine are used in such a case, there sometimes arises the failure of synchronizing of the generator. The starting methods of the cases I and II will be described with reference to FIG. 1. The case I refers to a route O-K-X-Y-Z, while the case II refers to a route represented by a curve 7. In either case, when r.p.m. approaches to the rated r.p.m., a variation in flow rate due to a governing action of a governor, and hence a water hammer action take place, presenting a difficulty in the governing action. As a result, a large variation in r.p.m. arises, thus disabling synchronizing of the generator.